The invention involves a procedure for positioning the actuating drive in a fuel injector and a device for performing the procedure in accordance with the generic part of claim 1.
Reservoir injection systems using very high injection pressures and high switching speeds are being increasingly used in the fuel supply of combustion engines. Such injection systems are known as common rail systems (for diesel engines) and HPDI injection systems (for Otto engines). In these reservoir injection systems, the fuel is fed into a high-pressure reservoir common to all cylinders. The fuel is then injected into the individual combustion chambers of the combustion engine by means of fuel injectors.
The fuel injector generally includes an injection valve that is opened and closed hydraulically by a servo valve in order to precisely set the timing of the injection process in the combustion chamber.
The servo valve is actuated by an electrically triggered actor. The use of piezoelectric actors has proven to be particularly effective in achieving sufficiently short switching times. In this kind of piezoelectric actor, longitudinal extension that is transferred to the servo valve which then again opens or closes the injection valve is brought about by the application of electrical voltage. For the longitudinal extension of the piezoelectric actor, which is in the xcexcm range, to be able to actuate the servo valve, this longitudinal extension is generally either mechanically assisted by lever gears with bearings in fuel or hydraulically amplified by a pressure chamber.
A fuel injector with a piezoactor and hydraulic amplification is described, for example, in U.S. Pat. No. 5,779,149.
In order to be able to attain the high switching speeds required for optimal combustion timing and small injection amounts with the fuel injector, it is necessary to adjust the fuel injector very precisely.
This applies particularly to the idle stroke between the piezoelectric actor and the servo valve. On the one hand, the idle stroke should be as small as possible to have constantly defined conditions and to keep the dynamic loads low. On the other hand, there must be minimal play between the actor and the adjusting element in order to avoid malfunctions during operation.
The setting of the idle stroke in the fuel injector has previously be done in such a way that the exact configuration of the individual components of the fuel injector and especially spaces between them are determined by computer from the dimensions of these components.
For that purpose, each component has to be measured at considerable expense. After measurement, the idle stroke is then set by adjustment disks placed between the injector housing and the actor or the servo valve; these disks must have only very close tolerances and are therefore very expensive to manufacture.
To check the adjusted idle stroke, it has previously been necessary to assemble the fuel injector completely and to test it under operating conditions. If malfunctions are found, the fuel injector must be completely broken down again into its individual parts after the test run and possibly reworked or the adjustment disks must be replaced.
It is the task of this invention to create a procedure for positioning the actuating drive in a fuel injector and a device for performing such a procedure in which it is possible to reliably position the actuating drive in the fuel injector at little expense and to allow operational production-line testing of the fuel injector.
The invention fulfills this task with the measures indicated in claim 1.
The above task is accordingly fulfilled in this invention by adjusting the idle stroke in such a way that a defined electrical voltage is applied to the piezoactor before it is mounted in the fuel injector so as to cause a longitudinal extension of the piezoelements that corresponds exactly to the desired idle stroke. In this state, i.e., with voltage applied, the baseplate of the piezoactor is surface ground with the actor housing. In the no-current or no-voltage state, the baseplate stands back from the actor housing by the idle stroke distance. The invention also provides a compensation collar between the piezoactor and the housing of the fuel injector. The compensation collar is inserted into the fuel injector that is now completely assembled up to the piezoactor, and is deformed by a prestressing device with a flat effective area until the servo valve lifts up from its valve seat. If, instead of the prestressing device, the piezoactor is then screwed into the injector housing to the stop on the compensation collar, the idle stroke set as indicated above is necessarily adjusted between the baseplate of the piezoactor and the adjustment element of the fuel injector servo valve. In this way an effective connection is formed between the piezoactor and the adjustment element for the servo valve with determined positions in such a way that the idle stroke between the piezoactor and the adjustment element always keeps the given value despite the unavoidable manufacturing tolerances of the individual components.
For the surface grinding of the housing and baseplate of the piezoactor, the latter is preferably clamped into a grinder, with the given voltage being applied via slip rings on the piezoactor. The compensation collar consists preferably of soft iron or soft copper. During prestressing, the material of the compensation collar flows, which permanently changes the thickness of the compensation collar.
Alternatively, the piezoactor can be surface ground before it is mounted, without voltage being applied. The idle stroke provided is adjusted via a boss on the prestressing device during deformation of the compensation collar. In this alternate embodiment, the prestressing device does not consist of a flat stamp but a stamp with an embossed face.